Combine all ingredients in a medium bowl. Cover and refrigerate 2 hours or overnight.
While the gravy simmers, skewer meat and fully cook on a grill or in a hot oven (see Notes for instructions on how to cook it in the oven).
Heat the ghee over medium heat in a large, deep-sided pan with a lid. Once hot, add the onion, bell pepper, salt, and black pepper, and sauté (uncovered) until starting to soften but not brown, about 5 to 7 minutes, stirring occasionally.
Add the jalapeno, garlic, and ginger, and sauté 1 minute, stirring constantly.
Add the bay leaf, 2 ½ teaspoons garam masala, coriander, cumin, and crushed red pepper flakes, cinnamon, cardamom, cloves, nutmeg, and fenugreek, and sauté until fragrant, about 30 seconds, stirring constantly.
Stir in the broth, tomato sauce, tomato paste, sugar, and lemon juice.
Bring up to a boil, cover the pan, turn heat down to low, and simmer until the vegetables are tender and the sauce is thickened, 20 to 30 minutes, stirring occasionally.
Turn the heat off and stir in remaining ½ teaspoon garam masala, cream, cilantro, and cooked lamb. Taste and adjust seasonings as desired.
Serve the lamb tikka masala topped with a sprinkle of fresh chopped cilantro if desired, alongside prepared basmati rice.
